The present invention generally relates to powder coating systems and, more particularly, to systems that induce a swirling flow of air and electrostatically charged powder about a workpiece.
Powder coating technology has generally evolved over several years into various types of coating techniques. Certain advanced techniques involve the use of electrostatic technology to adhere a powder, such as a resinous polymer or paint, to a desired workpiece preferably with a uniform thickness. The initial adherence of the powder to the surface of the workpiece takes place due to the attraction created by different electric charges existing on the workpiece and the particles of powder. After the initial electrostatic powder coating is formed, the coating is cured using techniques such as heat, infrared light or ultraviolet light, to fully adhere the coating to the workpiece.
Conventional electrostatic techniques for initially adhering the powder particles to a workpiece include two general types. The first type involves electrostatically charging the powder particles emanating in a cloud from a fluidized powder bed. When an electrically conductive, grounded workpiece is placed within the emanating cloud of electrostatically charged particles, the charged particles become attracted to the outer surfaces of the workpiece and form the initial layer of coating. The workpiece may be manually placed within a powder coating hood containing the electrostatically charged powder cloud or may be on a conveyor system or otherwise moved continuously through the hood or other powder coating area. The second powder coating technique utilizes a spray gun discharging electrostatically charged powder particles. An electrode at the gun nozzle may electrostatically charge the particles or the gun may emit a stream of powder particles charged by air upstream of the gun nozzle. With electrostatic guns, the workpieces are again grounded and typically placed within a spray coating hood during a coating operation to contain and collect excess sprayed powder.
A significant, continuing problem associated with electrostatically coating workpieces concerns achieving a uniform coating on the workpiece. With a workpiece oriented or moving horizontally, for example, this problem particularly exists with respect to top to bottom uniformity. That is, the lower surfaces of the workpiece tend to develop a heavier coating build than the upper surfaces. It is believed that this effect may be attributed to stratification, or a progressive decrease in density of particle distribution upwardly over the bed. Also, the charge on the particles may reduce in strength with increased remoteness from the voltage source and/or due to dissipation of the initial charge.
Various solutions have had different amounts of success in dealing with these problems. Some apparatus deal with these problems by creating a swirling or vortex-like flow of air and powder about the workpiece to more evenly distribute the powder particles on all outer surfaces of the workpiece. For example, U.S. Pat. Nos. 4,606,928; 4,808,432; and 5,773,097, each assigned to the assignee of the present invention, are all concerned with apparatus that distribute charged powder particles more uniformly about a workpiece, such as a continuous elongate strand of wire, cable, tubing or other like material. While devices such as these have met with significant success, it would still be desirable to provide improvements relative to achieving cost efficiencies, size reduction and reduced overall complexity of the devices as well as continued improvement in coating uniformity.
The present invention generally provides powder coating apparatus including a housing having a coating area positioned generally between first and second end walls with the first end wall having an inlet for receiving a workpiece and the second end wall having an outlet for allowing the workpiece to exit the coating area. A workpiece travel path extends between the inlet and the outlet and, for example, may generally define the travel path of an elongate wire, strand or other continuous or discrete workpieces moving along the travel path. In accordance with the invention, the coating area includes a surface extending lengthwise along the direction of the workpiece travel path and sloping transversely at least partially around the workpiece travel path. This transverse, sloped surface may be on an outer wall of the housing or may be a sloped interior wall or baffle structure in a conventional box-like housing or any other interior sloped surface achieving the effects of this invention. The surface is preferably sloped in at least two directions to promote an annular swirling air pattern as will be described below and is more preferably a continuously curving wall surface, such as a cylindrically-shaped wall surface. An air moving device is positioned to move air adjacent this surface to produce the swirling air pattern about the workpiece. In the preferred embodiment, the air moving device introduces pressurized air against the surface and lengthwise along the workpiece travel path. This surface may be referred to as a vortex inducing surface because the air follows the surface and generally flows in an annular swirling pattern around at least a portion of the workpiece travel path. A powder introducer is operatively connected with the coating area of the housing and introduces powder into the annular swirling pattern of air to coat the workpiece. It is contemplated that the air moving device could be a vacuum producing device and it is preferred that the powder introducer directs electrostatically charged powder into the swirling air pattern.
The pressurized air input device most preferably comprises a tube extending along the length of the vortex inducing surface and having a plurality of apertures or, for example, one or more slots for directing air into the swirling pattern. As one illustrative alternative, the pressurized air input device may instead comprise a plurality of separate air inputs, such as nozzles, mounted adjacent the vortex inducing surface. The charged powder introducer preferably includes a powder fluidizing bed communicating with the coating area and adapted to receive a supply of ionized or charged air to form a cloud of charged powder. A powder feed hopper may be provided to supply powder to the fluidizing bed. In this preferred embodiment, the annular swirling pattern of air also swirls the charged powder cloud generally around and into the workpiece travel path. As one illustrative alternative, the charged powder introducer may include a charged powder supplier, such as a triboelectric powder spray gun, connected with the pressurized air input device for supplying charged powder directly into the swirling pattern. In another alternative, a spray gun or other charged powder introducer may be otherwise connected with the coating area.
In another aspect of the invention, a pair of vacuum chambers may be connected proximate the inlet and outlet of the housing to prevent powder from exiting the housing. More specifically, one or both chambers may be connected to a source of vacuum in a manner that draws air and powder out of the respective chambers in a swirling pattern. When used in conjunction with the vortex inducing system associated with the coating area or another vortex system, the air and powder flow pattern in the vacuum chamber or chambers preferably swirls in the same direction as the swirling pattern in the coating area.
From the foregoing description, it will be recognized that a cost efficient, relatively simplified apparatus has been provided for inducing a swirling or vortex-type of charged powder and air flow around a workpiece for effecting uniform electrostatic powder coating. The use of inlet and outlet vacuum chambers also inducing a swirling pattern of powder and air further promotes a uniform coating.
Other embodiments of the invention also generally comprise a housing having a coating area for receiving a workpiece or workpieces and a workpiece travel path extending between an inlet and outlet thereof. In these alternative embodiments, first and second vortex generators are mounted around the inlet and the outlet and comprise inner and outer tubular structures each connected with a pressurized air inlet. The pressurized air inlet communicates with at least one annular space formed between the inner and outer tubular structures and the annular space opens around the workpiece travel path within the coating area. The air inlets are configured to introduce pressurized air in an annular swirling pattern within the annular space and out around the workpiece travel path. This may be accomplished using a generally tangential air inlet connection. As in the first embodiment, these embodiments will include a powder introducer operatively connected with the coating area of the housing for introducing powder into the swirling pattern of air.
In one alternative embodiment, the tubular vortex generators are connected to the outside of the housing and coating area while, in another alternative embodiment, the tubular vortex generators are connected within the housing and the coating area. It is contemplated that other modifications, such as partially mounting the vortex generators both inside and outside of the housing, or eliminating the inner tubular structure, may be used as well. As in the first embodiment, the charged powder introducer may comprise a powder fluidizing bed that produces a cloud of charged powder directed into the swirling pattern of air or a charged powder supplier, such as a triboelectric powder spray gun, operatively connected with the coating area. For example, the spray gun may be connected to one or both of the air inlets communicating with the annular spaces.
As further alternatives utilizing concepts in accordance with the invention, the inner tubular structures may be eliminated from the embodiments discussed above, and positive pressurized air may be introduced into a single tubular structure in a generally tangential manner to introduce a generally annular swirling flow within the tubular structure such that it becomes directed into the main coating area of the housing around the workpiece travel path. This may be used as the main vortex generator of the coater or may augment another vortex generator in the coater, such as the one described in connection with the first embodiment. As another alternative, the inner and outer tubular structures discussed above may instead have their annular space connected with a source of vacuum to act as powder collectors proximate the inlet and outlet of the coating area. In this option, a swirling vacuum effect is created in the annular spaces of the tubular structures preferably in the same direction as the main annular swirling flow in the coating area. The main annular swirling flow may be formed, for example, in accordance with the first embodiment.
The present invention further contemplates various electrostatic powder coating methods that may be carried out in accordance with the general teachings of the inventive concepts discussed herein. Additional objects, advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon reviewing the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.